This application claims priority under 35 U.S.C. .sctn..sctn.119 and/or 365 to Hei 9-155033 filed in Japan on Jun. 12, 1997, the entire contents of which are herein incorporated by reference.
1. Field of the Invention
The present invention relates to a mobile communication system that involves periodic switching of the transmitted frequencies by a frequency hopping technique.
2. Description of the Prior Art
A conventional mobile communication system will be described first with reference to FIG. 12 which depicts a conventional system in conceptual form.
Reference numeral 10 denotes a mobile station, and 11 a base station. Data is transmitted and received between the base station 11 and the mobile station 10. Reference numeral 12 denotes a radio cell, which indicates the coverage area surrounding the base station 11 and within which data is exchanged between the base and mobile stations 11 and 10.
A septuplet of radio cells 12 constitute one area as indicated by hatching in FIG. 12. The frequencies that are assigned to each of such areas repeat in a cyclic order; that is, the areas are common in the frequencies assigned thereto. Each area made up of seven radio cells will hereinafter be referred to as a repetition area. Further, the number of radio cells forming the repetition area will hereinafter be referred to as a repetition cell number. In the prior art example of FIG. 12, the repetition cell number is seven.
Furthermore, the radio cells forming the repetition area are each assigned a plurality of frequencies, which will hereinafter be referred to as a frequency group. And the radio cells making up the repetition area are each assigned seven different frequency groups.
The base station 11 has control over the mobile stations 10 belonging to its radio cell and allocates to each mobile station 10 a hopping sequence for frequency hopping use.
Next, a description will be given of the operation of the conventional mobile communication system conceptually depicted in FIG. 12.
In FIG. 12 the base station 11 specifies predetermined hopping sequences for all the mobile stations 10 placed under its control. Based on the hopping sequence specified by the base station 11, each mobile station 10 performs frequency hopping.
Now, a description will be given, with reference to FIG. 13, of hopping sequences c1, c2, c3, c4, c5 and c6 that are specified for a cell A or B when it is assigned frequencies f1, f2 and f3. Let it be assumed that mobile stations M1 to m3 are present in the cell A and mobile stations M4 to m6 in the cell B.
Suppose that the mobile station M1 is assigned the hopping sequence c1, the mobile station m2 the hopping sequence c2, the mobile station m3 the hopping sequence c3, the mobile station M4 the hopping sequence c4, the mobile station m5 the hopping sequence c5 and the mobile station m6 the hopping sequence c6. The mobile stations M1 to m6 perform frequency hopping with fixed hopping periods based on the hopping sequences c1 to c6 respectively assigned to them. The frequencies f1, f2 and f3 will hereinafter be called first, second and third frequencies in this order.
Turning next to FIG. 14, a description will be made of how frequency hopping takes place, for example, in the mobile stations M1 to m3 in the cell A. The abscissa represents time.
The mobile station M1 uses the frequencies f1, f2 and f3 when it outputs bursts B1, B2 and B3, respectively. The mobile station m2 uses the frequencies f2, f3 and f1 when it outputs bursts B1, B2 and B3, respectively. The mobile station m3 uses the frequencies f3, f1 and f2 when it outputs bursts B1, B2 and B3, respectively.
The bursts B1 to B3 mentioned herein are pulse-like waves or waveforms created by dividing original data such as speech and adding a header or the like to each divided piece of data. By virtue of the hopping sequences c1 to c3 assigned thereto, respectively, the mobile stations M1 to m3 in the cell A will not be assigned the same frequency at the same timing. Likewise, the mobile stations M4 to m6 in the cell B will not be assigned the same frequency at the same timing.
FIG. 15 depicts in block form the mobile station 10 that forms the conventional mobile communication system. With reference to FIG. 15, the configuration of the conventional mobile station 10 will be described.
In FIG. 15, reference numeral 20 denotes an antenna. Reference numeral 21 denotes a transmitting part, which is connected via a switch 22 to the antenna 20. Reference numeral 23 denotes a receiving part, which is also connected via the switch 22 to the antenna 20. Reference numeral 24 denotes a transmission/received data processing part, which is connected to the transmitting part 21 and the receiving part 23. Reference numeral 25 denotes a synthesizer part, which is connected to the transmitting part 21, the receiving part 23 and the transmission/received data processing part 24. Reference numeral 26 denotes a control part, which is connected to the transmission/received data processing part 24 and the synthesizer part 25.
Reference numeral 24i denotes transmission data, which is input into the transmission/received data processing part 24. Reference numeral 24t denotes transmission burst data, which is provided from the transmission/received data processing part 24 to the transmitting part 21. Reference numeral 21t denotes a transmission signal, which is output from the transmitting part 21. Reference numeral 23r denotes a received signal, which is input into the receiving part 23. Reference numeral 24r denotes received burst data, which is provided from the receiving part 23 to the transmission/received data processing part 24.
Reference numeral 24o denotes received data, which is output from the transmission/received data processing part 24. Reference numeral 25t denotes transmission frequency data, which is provided from the synthesizer part 25 to the transmitting part 21. Reference numeral 25r denotes receive frequency data, which is provided from the synthesizer part 25 to the receiving part 23. Reference numeral 26r denotes hopping sequence data, which is provided from the transmission/received data processing part 24 to the control part 26. The transmission/received data processing part 24 instructs the control part 26 to perform frequency hopping based on the hopping sequence data 26r.
Reference numeral 26t denotes first switching data, which is provided from the control part 26 and fed into the synthesizer part 25. The synthesizer part 25 operates on the first switching data 26t, performing frequency hopping. Reference numeral 24g denotes a timing signal, which is provided from the transmission/received data processing part 24 and fed into the synthesizer part 25. The timing signal 24g is output from the transmission/received data processing part 24 based on the received burst data 24r.
Next, the operation of the conventional mobile station 10 will be described below.
The mobile station 10 receives signals sent from the base station 11 by the antenna 20. The received signal 23r is applied from the antenna 20 to the receiving part 23 via the switch 22. When supplied with the received signal 23r, the receiving part 23 demodulates it and outputs the received burst data 24r, which is fed into the transmission/received data processing part 24.
The transmission/received data processing part 24 decodes the received burst data and outputs the received data 24o. Further, the transmission/received data processing part 24 extracts the hopping sequence data 26r from the received burst data 24r. The transmission/received data processing part 24 applies the timing signal 24g to the synthesizer 25. Additionally, the transmission/received data processing part 24 is supplied with the transmission data 24i and generates therefrom the transmission burst data 24t, which is fed into the transmitting part 21. The hopping sequence data 26r output from the transmission/received data processing part 24 is input into the control part 26.
Based on the hopping sequence data 26r input thereinto, the control part 26 determines the frequency desired to hop. And the control part 26 uses the first switching data 26t to indicate the thus determined hopping frequency to the synthesizer part 25. The synthesizer part 25 further receives the timing signal 24g from the transmission/received data processing part 24, and determines the frequency to hop and its timing.
Based on the first switching data 26t fed thereinto, the synthesizer part 25 provides the transmission frequency data 25t or received frequency data 25r to the transmitting part 21 or receiving part 23 so that the frequency to be used by the mobile communication system equipped with the synthesizer part 25 is switched, for example, from a first to a second frequency.
When the synthesizer part 25 indicates a predetermined frequency to the transmitting part 21, the former provides the transmission frequency data 25t to the latter. When the synthesizer part 25 indicates a predetermined frequency to the receiving part 23, the former provides the received frequency data 25t to the latter. The transmission burst data 24t provided from the transmission/received data processing part 24 is input into the transmitting part 21. Supplied with the transmission burst data 24t, the transmitting part 21 modulates it based on the transmission frequency data 25t and outputs the transmission signal 21t. The transmission signal 21t provided from the transmitting part 21 is sent to the base station 11 via the switch 22 and the antenna 20.
With the conventional mobile communication system described above, frequency hopping is performed in a predetermined cycle regardless of the receiving conditions of the mobile station. Hence, a poor receiving level may sometimes be switched to a high receiving level, but the cyclic or periodic frequency hopping gives rise to a problem that a high receiving level may be switched to a low level. Another problem of the prior art is that no particular attention is paid to interferences.